Time division multiplex system utilizing a step-wave generator in the distributor circuit



July 29, 1952 B. TRI-:VOR 2,505,360

TIME DIvIsIoN MULIIPLEX SYSTEM UIILIZING A STEP-WAVE GENERATOR IN THE DISTRIBUTOR CIRCUIT l0 Sheets-Sheet 1 Filed March lO`. 1947 vm. mm.

B. TREvoR TIME DIVISION MULIIPLEX sysTE GENERATOR IN THE DIST July 29, 1952 Filed March 1o, 1947 July 29, 1952 B. TRI-:VOR 2,605,350

TIME DIVISION MULTIPLEX SYSTEM UTILIZING A STEP-WAVE GENERATOR IN THE DISTRIBUTOR CIRCUIT Filed March 10, 1947 l0 Sheets-Sheet 5 INVENTOR 3) N Mmmm TRsvoR Q x 'Bv/Wm fm F/a. t.

U.\ ATTORNEY.

July 29, 1952 B. TREvoR 2,605,360

l TIME DIVISION MULTIRLEX SYSTEM UTILIZING A STRP-WAVE GENERATOR IN THE DISTRIBUTOR CIRCUIT FiledvMaroh l0, 1947 10 Sheets-Sheet 4 /l/JZ fit', mail. ,4 /r I RR' 00, n n A n n l v v v v u uI v 99 :of o I Il I' {02 l I lq +2201 l t l! I@ f/ K +/Z0u S 0 /03 um :7" l r 20v II A f"1 A fg@ -0 los abou! V V 301/ 4 r I :hv/40a :fr nl o [06 H H u t o IOT G M g o l INVENTOR BERTRAM TREVOR ATTORNEY July 29, 1952 Filed March l0, 1947 Kal rmyf B. TREVOR TIME DIVISION MULTIPLEX SYSTEM UTILIZING A STEP-WAVE GENERATOR IN THE DISTRIBUTOR CIRCUIT (wwf/v r WwW/5 l0 Sheets-Sheet 5 INVENT'OP` BERTRAM TREVOR l BY AATTORN EY B. TREVOR 2,695,360 TIME DIVISIGN IVIULTIPLEX SYSTEM UHLIZINI'; A STEP-WAVE GENERATOR IN THE DISTRIBUTOR CIRCUIT lO Sheets-Sheet 6 Filed March lO, 1947 IIIIIIIIIIIIIIIIIIIIIIIIIIIIIII IIMMMSMQWMIIIIIIIIIIIIII AT TORNEV July 29, 1952 Filed March 10. 1947 l TRI-:VOR TIME DIvIsIoN MULTIPLEx SYSTEM UTILIZING A STEP-WAVE GENERATOR IN THE DISTRIBUTOR CIRCUIT 10 Sheets-Sheet '7 das @miauw/Mmm AMT/rj@ INVENTOR BERTRAM msvon ATTORNEY `uly 29,

Filed March lO, 1947 Fiyi. I

1952 B. TREVOR TIME DIVISION MUIIIPIEX SYSTEM uIILIzIN-G A STEP- w GENERTGR IN THE DISTRIBUTOR CIRCUIT lO Sheets-Sheei 8 AVE /lf/LLLEIIII;

BERTRAM TREVOR ATTORNEY July 29, 1952 B. TREVOR 2,605,360

TIME DIVISION MULTIFLEX SYSTEM UTILIZING A STEP-WAVE GENERATOR IN THE DISTRIBUTOR CIRCUIT Filed March 1o, 1947 1o sheets-sheet 9 IN BERTRAM TRM-'0R TTORNEY STEP-WAVE T lO Sheets-Sheet l0 R ma O EV I I I Mw Pic G0 0 .SRSQQQ u @www m w QT .waw n f H Qmwm W.. m \W n. I" Mm. mw) m w: bb i B. TREVOR LEX SYSTEM UTILIZING A THE DISTRIBUTOR CIRCUI AAA AAAAA GENERATOR IN TIME DIVISION MULTIP 1 i i N E;

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`Iuly 29, 1.952

Filed March Io, 1947 I ESAN* Patented `Iuly 29, 1952 TIME DIVISION MULTIPLEX SYSTEM UTI- LIZING A STEP-WAVE GENERATOR IN THE DISTRIBUTOR CIRCUIT Bertram Trevor, Riverhead, N. Y., assigner to Radio Corporation of America, a corporation of Delaware Application March 10, 1947, Serial No. 733,697

4 Claims.

This invention relates to multiplex communication systems, and particularly to such systems utilizing pulses of electrical energy.

An object of the present invention is to provide in a multiplex system a plurality of differently biased channel selectors which are fed in parallel with a step voltage wave, whereby the channel selectors become effective at different steps or risers of the applied step voltage wave.

Another object of the present invention is to provide a method of and apparatus for sequentially assigning equal time intervals to a plurality of channel circuits by means of a step voltage wave generator.

A further object is to provide a means to allow a simple, easy and eiective method of adjusting a plurality of channel selector circuits in a multiplex system to become sequentially eiective at different non-overlapping time intervals.

An important feature of the present invention lies in the use of the step voltage wave generator in a time division multiplex system for controlling a plurality of channel circuits. Since the step voltage wave is characterized by precise timing of its risers, this inherent advantage is utilized to determine precise sequential intervals of time for the different operating periods of the separate channel circuits.

In the accompanying drawings:

Fig. 1 illustrates, in box form, the complete transmitting system for a pulse type multiplex communication system in which the invention is employed;

Figs. 2 and 2a, taken together, schematically illustrate the circuit details of the transmitter of Fig. l;

Figs. 3 and 3a, taken together, show a series of curves graphically illustrating different voltage wave forms appearing in different parts of the transmitter;

Fig. 4 illustrates, in box form, the complete receiving system for a pulse type multiplex communication system in which the invention is employed;

Figs. 5 and 5b, taken together, are a series of curves graphically illustrating different voltage waveforms appearing in different parts of the receiving system;

Fig. 6a. illustrates the circuit details of the apparatus coupled to the output of the superheterodyne receiver and shown in Fig. 4 as the common unit; and

Fig. 6b illustrates the circuit details of the apparatus constituting a channel unit of which 2 there are as many as there are channels in the system.

The transmitter of Figs. l and 2, 2a is used in an eight channel multiplex system and utilizes short pulses of radio frequency energy which are time displaced by modulation. For multiplexing purposes, the pulses corresponding to the separate channels are separately and consecutively generated at a fixed repetition rate which will be called hereinafter the synchronization rate; corresponding to a iixed time interval to be called the synchronization period. In this transmitting system the synchronization rate is 10 kc. and the corresponding period 100| a sec. (microseconds). A pluse occurs once each synchronization period for each channel. The individual rates and periods are consequently the same and equal to the synchronization rate and period. Each channel pulse occurs at a rate of 10 kc. and the separation betwen adjacent pulses in each channel for an unmodulated condition is 100 a sec., or microseconds. Because the unmodulated signal pulses are similarly located in each channel, they are, therefore, about 11.1 microseconds apart in the common output circuit. The pulses in each channel can be modulated I 4 ,a sec. (peak modulation), thus leaving a guard space between pulses from succeeding channels of about 3.1 ,c sec. This guard space is necessary to reduce cross modulation defects. The pulses from other channels occur in the interval between adjacent pulses from any one channel. rlhe synchronization pulses occupy the ninth interval of 11,1 microseconds. The output pulses from the channels are of constant length and the time between two adjacent pulses is measured from the leading edges.

In Fig. 1 there is shown a 90 kc. crystal oscillator producing pulses of current which feed into and lock by injection a kc. short pulse oscillator I2. The short output pulses from the pulse oscillator I2 are applied to a counter or step-wave generator I4. The voltage wave forms from the apparatus I0, I2 and I4 are illustrated by the curves 9, I I and I3, respectively, appearing immediately above the equipment.

The counter I4 provides two outputs, one of which is the step wave I3 which is supplied to the coupling amplier I6 and the other of which is a synchronization pulse occurring once for each step wave cycle and which is applied via lead I5 to a synchronization pulse generator 30. The function of the step wave I3, which is applied to the coupling amplifier and then to the different channels over lead Il, is to time the channel selector I3; a pulse generator 22; and p an output circuit including a Shaper' or clipperlimiter 24. The pulses producedA by the channel pulse generator 22 are modulatedl as to time or phase by means of a modulator 25 which is supplied with suitable signals, such a-s speech'. fromv an audio ampliner 28. All channels have their inputs connected together in electrically parallel relation.

The channel selectors are differentially self,-

biased and each channel selector is normally biased Well beyond the current cut-oli condition. The bias of each channelV selector is so'adjustedv that the applied step Wave from the coupling. amplier it causes current to nou/consecutively inthe different channel selectors. @ne channel selector conducts for each rise of voltage in the step wave I3 up tov eight, Whichis the number ofA channels'. Each step rise in theV step wave is vgreat enough to insurethat during its occurrence the current of the correspondingly biased channel selector shall be driven rapidly from beyond the-'cut-oil` condition to.- a zero' bias value. Once achannelselector starts to conduct, the current new therein Will continue untilV the' end of the a ratioof synchronization period, When theinput voltage Y toy ther selector drops to zeroY at the -end of the step; The outputs from all channelsV appearing in leads 2li-'flow in a common lead 3:! to differentiator and clipper circuits 32 and 34 from which' pulsesorv shorter: duration are applied to a power amplifier Whose output controls they production of radio frequency pulses from a magnetron' oscillator 4i). The very short duration output pulses from magnetron lll which may each have an effective duration of 0.3M sec., are fed. to antenna t2', from which they are radiated to aY remotely located`v receiving terminal (not shown). The synchronization pulse generator 3l! .which receives alpulse over lead iol from'y the counter Meat the end of each step Wave, produces ai pulseA at the end. of each step wave Which-is. supplied Y tothe diiiferentiator and clipper 34 and also fed..

to the amplifier 36 and/'magnetron 49 together 'with the channel pulses. V'llherewill thus be eight consecutively appearing pulses from the eight" different channels followed by'a'. synchronization pulse for each cycle of operations; Eorthe une modulated condition, all of these channel rpulses, and they synchronization pulse. will be separated from oneanotherV by a spacing-equivalent to` 11.1@V seo. it'. will. thus be4 seen-that the synchronization periodof 100;; sec. is divided into nine equal:

intervalsA by the step counterl or step 4wave gen,-

erator I4, and that all of these pulses aresini- Y ilarly located in each 'oneof these ninevequal'.

intervals and similarly vspaced-apart for the unmodulatedv signal condition. i Y Y.

1n the schematic illustration of Figs. 2v andlZd,

the circuitswhich correspond with the circuits;

of Fig. l have been given the same labeling and reference numerals. The 9,0 lko. crystal oscillator I0 has an electron coupled anode circuitand supplies pulsesof rcurrent'tocontrol the 90 kc'. pulse oscillator I2. The pulse frequency from the oscillator I2is normally determined in the main by the time constantsv of 'R' andV C, inthe absence of the applied synchronizing Wave from Y the crystal oscillator. In practice, the values of R and C are so chosen that the frequency of the pulse oscillator is slightly lower than the kc. crystal oscillator I0. Transformer TR is a low inductance, tightly .coupledarrangement. The: sharp 'output pulsesfromfther 90` kc. Vpulse oscillator I2 are each about 1u sec. longrand have about 40 volts amplitude. These pulses are applied to the condenser CI o the counter cir- Y cuit I; This counter circuit or step Wave generator comprises-a condenser CI, a pair of diodes DiV and D2 (which may be arranged Within a single evacuatedy envelope or comprise two separate tubes as shown), a condenser C2, and normally non-conductive triode vacuum tubes A, B and E. Condenser CI is of much smaller value than condenser C2 and in the present case (Where a step wave voltage output of nine steps is desired) where condensers may for example, have For the positive vrising edge of a pulse from the pulse oscillator LI2, diode DI will not-conduct,

but D2" will conduct; and hence the circuit Willappearv as though' the two condensers CI`VA and C2 are in series to ground. HenceA 1/20 or during the negative drop. During the nextl pos'-A itive rise Aof the'applied pulse,v thecondenser CI will be recharged'through D2.' Each time there is a positive ris'e'involtage appliedby thefpulsej oscillator to the condenser CI. there `will be anr incremental' increase or step-'up in voltage Von condenser C2k of about 20A volts, although each charge on condenser C2 after the'iirst is slightly less than the preceding one.r Zitshould beY im-y portant to note at this timer that there is no re` sistance established across condenser C2 because it is desired that there be complete absence of current or charge leakage on this condenser during the voltage step-up operation.

v Vacuuin'tubes A and B and are normally,"

non-conductive; that is, they are biasedto'cutoff condition. TubeA has a volt bias on its cathode, and hence the voltage across condenser C2 must exceed approximately +155 voltsb'efore" tube A conducts. The buildingupofa` charge on C2on the'-4 ninth rise to about 155, volts will cause tube A toesta'rt 'conducting'very suddenly,-

as a result ofwhich a pulse of current is passed 'throughv transformer T2, which isso poled that it appliedv av sharp positive pulse'l to the gridv of` tube B, vcausing tube B to conduct suddenly andA l' pass a sharp pulse to the grid of tube E via lead"V 58.

oscillator-.andi is connected regeneratively to pro-U 'llube Bi, inl effect, is anv 'over-biased pulse duce only lone pulse in responseto ther flow of 4 current in tube A, after which it ceases conducting. Tube B triggers oii or res to produce'a constant amplitude'discharge pulse irrespective ofthe amplitude of the pulse from tube A, andV this discharge pulse causes normally non-con'- ducting tube E to' conduct. When tube-E conducts-it provides a lowimpedance path for'the charge on condenser C2. This condenser C2 then discharges through tube E to a relatively low value; The time constants of the counter or step` generator I4 are so designed that there are nine risers or incremental steps in voltage on C2 before it discharges. The counter provides two outputs, one a step wave voltage which is taken from condenser C2 and applied to lead 5|, and the other a synchronizing pulse at the end of each step wave which is taken from the transformer T2 and applied to lead I5. The cycle of operation then repeats itself. The counter |4 can be arranged to provide a different number of steps by varying the ratio of the capacitance values of condensers CI and C2, if a different number of channels are employed.

The coupling amplifier I6 is merely a cathode follower which obtains a step wave voltage input from the counter circuit over lead 5I. This coupling amplifier provides an output from its cathode over lead 52 of the same polarity and voltage as the input wave taken from condenser C2, without loading condenser C2. Amplifier I6 does not draw grid current.

Output from the coupling amplifier is taken from its cathode and is applied to all of the channel selectors I8 in parallel. The apparatus in only one channel has been shown since all of the channels have substantially identical equipment and function similarly. Each channel includes a channel selector I8 and the different channel selectors are biased differently by individual cathode rheostats 53, each of whose effective Values determines a certain cathode bias. Each channel selector includes a vacuum tube E0, a series grid resistor 49, an adjustable cathode bias resistor 53, a pair of resistors 55 and 58 connected in series between the anode of tube 60 and the source of anode polarizing potential B, a bleeder resistor 51, and a pair of by-pass condensers 59, 54.

The channel selectors are normally non-conductive; i. e., biased to cut-off. Channel number I is biased to conduct on the first rise in the applied step wave voltage, let us say at +35 volts, because the system is so arranged that condenser C2 cannot discharge below let us say 30 volts. The channel selector in channel number 2 is arranged to conduct on the second rise at another l5 volts higher then the bias on the first channel selector; or at a value of 50 Volts. The succeeding channels are arranged to conduct sequentially at different rises in the step wave voltage applied to the different channel selectors I8 by the coupling amplifier I6. In the channel number three, current will flow in the associated channel selector cn #3 riser in the step wave, while in channel #6 current will start to flow in its associated channel selector on #6 riser in the step wave. It will thus be seen that the step amplitude at which each channel selector conducts is determined by the cathode bias which is developed across resistor 53 and condenser 54, and that different adjustments of resistor 53 will give different cathode biases.

After each channel selector I"8 which is normally non-conductive starts to conduct, further rises in the step wave will not produce any appreciable anode current increase in that channel selector because of grid current limiting due to resistor 40. Stated in other words, each channel selector is caused to carry current starting on the particular step riser which furnishes a voltage of suflicient magnitude to overcome its vcutoff potential, and this channel selector will continue to pass current -to the end of the step wave voltage. Let it be assumed that channel selector |8 for the first channel starts to conduct on the first rise in the step voltage at a value of 35 volts. When the second rise in the step wave voltag-e occurs, the first channel selector is already passing current. The increase in voltage in the step voltage wave appearing in the output of the coupling amplifier will merely cause the grid of the first channel selector to draw current, and the channel selector thus acts very much like a diode between its grid and cathode to cause anode current limi-ting.y Hence, as each additional channely selector after the first starts to conduct (when additional rises occur in the step wave) the preceding channel selectors which are alreadyconducting will continue conduc-ting with substantially the same anode current as before. After the ninth rise in the step wave voltage, the amplitude of the step wave falls down to its minimum value. (30 volts in this case) and all channel selectors will cease conducting. This cycle of operations will repeat itself for each new step wave voltage.

The operation of the system so far described in connection with circuits I0, I2, I4, I6 and I8 may be better understood by reference to the curves |00 through |04 of Fig. 3. Curve 99 illustrates the sine wave output of the kc. crystal oscillator |0. Curve |00 illustrates the current pulses from the anode circuit of the crystal oscillator I0. Curve |01 illustrates the short sharp pulses in the output of the 90 kc. pulse oscillator I2 which are synchronized by the sine wave oscillator. Curve |02 shows the step wave voltage produced by the counter I4. It should be noted that this step wave voltage begins at about. 30 volts positive which is the lowest voltage to which condenser C2 c-an be discharged, and rises to about volts positive. The step wave voltage of curve |02 is shown as having nine steps or risers. Curve |03 shows the anode voltage of one channel selector |8, let us say in channel number 4, while curve I 04 shows the anode volta-ge of selector I8 in channel #L Thus, referring to curve |03, it will be seen that the anode voltage of the vacuum tube 60 of the channel selector is a maximum over a period t corresponding to the non-conducting period of the channel selector. and that this anode voltage drops suddenly on the fourth rise of the step wave volta-ge curve |02 when the channel selec-tor becomes conducting, and this anode voltage remains low over a period t corresponding to the conducting period of the channel selector of channel number 4. Similarly, -by referring to curve |04, it will be seen that the anode voltage on the channel selector vacuum tube is a maximum `over a period t corresponding to the non-conducting period of the selector tube, and drops suddenly at the first rise in the step wave voltage of curve |02, at which time the channel selector in channel #I becomes conducting. During the conducting period t of the channel selector I8 of channel #L the anode voltage on the channel selector tube will remain low. It should be noted from curves |03 and |04 that once the channel selector in a particular channel becomes conductive it continues to be conductive to the end of the step wave of curve |02.

The'channel selector |8 is so arranged that the amplitude of the output voltage pulse on the anode of its vacuum tube B0 is constant regardless of its particular channel position; or, putting aeoaeco' it inA other Words, thei diiterent channeliselectors furnish al constant amplitude output voltagev over a Wide range of cathode biases for the diflerent channels without requiring additional. adjustments when its bias is.V changed over this range. The operating positionoi any one channel selector circuit l relative to the different risers. on the step: Wave voltage'canbe changed by asimple bias adjustment Without the need for making other t'o compensatelfor the change. in biais.. The reason for this will now be given.

If'resistor 55 in thechan'nelI selector-'were zero; then! the voltagel :at point fwouldzbei constant (equal to +B) regardless of channel position', but the anode-to-cathode'voltage ofl the' vacuum. tube 60 prior to its conducting timeV wouldI decrease With increasing channel number, sincethe voltage at the cathode increases as thechannel number= is increased, as a result'of' Whichthe output voltages for the different' channelselectors'would be different. Whenresistor 55 is given a;r finite' value, then the voltage at'pointv 56zover the entire time for one complete step Wave (considering only one cycle of operations) increases inthe differentv channels with increasing channel number, since the average current in vacuumv tube 60 decreases over this same timeA interval. By properly choosing a value of +B andl resistor 55, the anode voltage can 'bev made cons-tant; With channel position for all channels; that. is', changes of voltageat point 56' will bev equal to changes in the cathodev voltagev as the channel position is changed, resulting in an amplitude'of voltage pulse at the anode of tube 6D which is constant with channel position', since the peak anode current is a function of the anode to cathode-voltage.

It should be evident that the resistors 55l in the different channelsV carry different average currents, due to the different conducting periods of the channel selectors. Thus different average currents produce different IR drops in the resistors 55 to compensate fory different rising cathode voltages, but the voltage output from the dif-l ferent channel selecto'rsare practically the same since the cathode to platevoltageremains constant. Hence, one channel can be used inY another channel position Without making adjustments to compensate forthel different cathode biases. The same value of resistor' 55 is usedA in all channel selectors. Except for the different adjustments in cathode biases, all of the channelcircuits are identical. x f* A resistor 51 is employedl in the channelv selector'to'set the mean value of thevoltage at point l l 56 in order to provide the proper' operating conditions for vacuum tube Se. The use of this resistor 51 eliminates the necessity for. varying the value of +B inA order to obtain al suitable voltage for all channels. rThe resistor 51 serves to bleed current' from the vanode supply. Thisv resistor 51' has the same adjustment or valuev 'in all channels. Condensers 54 and 59 are, 'oy-pass condansers which. by-pass the alternating current component of anode current at the input Vfrequency. Resistor 49'is` a serieslgridresistor which; as mentioned before, serves to limit the'grid voltage tozero regardless of the'applied voltage- Although the channel selector I8 of the il vention has been shown as having a resistorv 51 shunting the by-pass condenser 59, it should be understood that. if desired, the resistor 5115 can be omitted, although this isA not a preferred arrangement. To achieve the sameresults with'. the resistor 51 omitted. the value of B would have' to 8 be suitably selected in order to obtain aY desired voltagefor all channels which would result inconstantiamplitudeof voltage pulses from the channel selectors. The use of resistor 51, however,- eliminatesA the necessityfor varying the value of B tofobta'in the desired voltage. I

'Ihe output from each channel selector is in the formof arectangular wave pulse which is caused by the drop inanode voltage in the vacuum' tube (ilk` This'. rectangular wave output pulse is appliedl to the saw-tooth generator 20 and is differentiated by condenser 6| and resistor E2. The result-of. this differentiation by condenser 6| and resistor GZSi'Sasharp negativeinput which is applied to the grid of vacuum tube 9|. Tube 9| is' nor.-

vmally conducting. The sharp negative impulse applied toits-grid from the differentiator 6|, 62 is ofA sufficient magnitude to bias the tube 9| Vto cutofiv and beyond for the duration of the negative impulse. The duration of this negative impulse is determined: by' the values of resistor 52 and condenser 6| and the. amplitude of the rectangular pulsek from tube 6G'. Since the amplitude. of this rectangular pulse is constant in accordance with 'z the invention, the duration. of this negative impulse is determined solely by the values of resistor 62 and condenser 6|. l

During' the time tube 9|A is normally conducting, the voltage on its anode is low. The application of thenegative impulse to the grid of tube 9| cuts off the flow of current in tube. 5|. and causes the anode voltage on this'v tube to rise, as a result of which condenser 63 starts to charge through resistor 64. When. tube 9| again conducts at'the .'1 end of: the applied negative pulse, condenser E3 discharges through the low. impedance Vpath of tube 9| to its normalr voltage. The result' is a short saw-.tooth wave obtainable from condenser 63 of a duration equal-.to theduration of thevnegative impulse applied tothe grid of tube 9|.. .In practice, this saw-tooth Wave maybe 10` mu sec.

, long, which is roughly ofthe order of a one channel'interval, or the length of one tread of the step wavefvoltage (curve |02 ofV Fig. 3).

Curve |65 of Fig. 3 illustrates the wave form of the differentiated pulse in channel #d which is appliedV tothe grid of tube 9|.` The interval t2 represents the non-conducting time duration of tube 9|, corresponding to the' modulation. rangeof channel #4.

The horizontal dash line in curve represents thecut-off grid voltage of the tube 9|. Themaximum negative value of thediferentiatedpulse is about v-30'volts.

The saw-tooth Wave produced by saw-tooth `generator 2Q' in the output of tube 9| is applied'to to -theoutput of the saw-tooth generator 20,. is

normally non-conducting and is so i biased as. to start' tov4 conduct at about the middler of the` appliedsaW-tooth Wave in the absence. of modulation. Coil 6'6, the output of pulse generator vacuum tube 22,A hasvoltages. ofA relatively opposite polarity onits end terminals when a rapid current change occurs. in it. 'This coilzi's' a dififerentiating coil WithA- distributed capacity. Damping resistors 61 are provided for both halves 9 ofthe coil 66 so as to limit vthe oscillations in the coil 66 to a single half period for each separate change of current.

Curve |01 in Fig. 3 illustrates the wave form of the anode current in a pulse generator 22 of channel #4 for the unmodulated condition. It should be noted that the pulses in curve |01 commence at about the middle of the applied saw-tooth wave of curve |06. Curve |08 in Fig. 3 represents voltage pulses in the output of the pulse generator 22 at point 68, due to the presence of current pulses of curve'I 01.

Modulation is applied to the vacuum tube of pulse generator 2:2 by modulator tube 26 which serves to vary the bias on tube 22 and hence to vary the critical value or time at which the pulse generator 22 starts to conduct, over the range of the applied saw-tooth.

Vacuum tube 69 is a cathode follower, as a result of which any variation in its grid voltage produces an equal voltage variation on its cathode. Resistor 10 carries the cathode currents of tubes 22, 26 and 69. Resistor 10 is a common cathode resistor for both vacuum tubes 26 and 69. A variation in current through tube 69 in response to modulation or rectified ringing causes a variation in the cathode voltage of tube 69. The grid voltage of tube 26 is xed and hence the variation in cathode voltage of tube 69 changes the average anode response of the modulator 26 and hence the cathode bias of the pulse generator 22. Any appreciable ilow of current through tube 22 must be through the modulator 26. A very large resistor 1| connected between ground and the cathode of pulse generator 22 (for example 2 megohms) is provided to prevent the omission of a pulse through the vacuum tube 22, in the event the modulator tube 26 is cut off by excessive modulation.

A failure to pass a pulse over each channel at all times for each assigned period may result in cross-modulation or cross-talk in the channels due to a change in power levels throughout the system. Each pulse draws about two amperes of current in the radio frequency transmitter and hence the failure of a pulse will change the average current in the various circuits in the transmitter and tend to produce cross-modulation or vinteraction of the channels. The use of resistor 1| prevents the drop out of a pulse in the event of the foregoing circuit conditions and assures the passage of a pulse from the pulse generator for each channel at all times during the assigned periods.

Only the front edge of the positive pulse passed by pulse generator 22 is ultimately used, and this front edge varies in time over the selected range in accordance with the modulation.

The dotted line pulses in curve |08 on opposite sides of the solid line pulses of positive polarity represents the extreme position during modulation. This extreme position is shown as covering a range t3, which is about i4 a sec. on each side of the solid line pulse, or 8 n sec. total.

The maximum advanced position in time due to negative modulated voltage is determined by the minimum anode resistance of modulator tube 26 (corresponding to maximum conduction position), which occurs when tube 69 is cut off, and the actual value of this minimum anode response is set up .by the values of resistors 10 and 12. When a maximum positive modulating voltage is applied to tube 69, its current will cut-off tube 26 and leave resistor 1| as the biasing resistor for tube 22 of the pulse generator.

10 l Thus, the flow of current through resistor 1| when tube 26 is cut olf automatically biases the pulse generator 22 so that it yields a useful pulse at the top position of any saw-tooth wave applied to the grid of tube 22.

Resistor 62 in the saw-tooth generator 20 may be called the modulation limit control because it varies the duration of the negative pulse in line |05 (Fig. 3) and hence the time during which tube 9| of the saw-tooth generator is cutoif. For this reason, resistor 62 is shown as being variable.

Speech and ringing are applied to leads 13. Ringing is furnished by a twentycycle low frequency audible source (not shown). The condenser 14 in the input circuit is a low reactance for the higher speech frequencies and is a relatively large reactance for the low ringing frequency. During speech, the modulation is applied to the grid of tube 69 via transformer T3 and lead 15. When ringing is applied, there is developed a voltage across condenser 14' which is applied to lead 16 to be rectified by tube 11, to thereby develop a direct current voltage across the resistor 18, such that a negative polarity appears on that terminal of resistor 18 which is connected to the grid of tube 69. Tube 69 is cut oil by this voltage of negative polarity resulting from ringing, and will allow modulator tube 26 to assume its minimum anode resistance for the flow of current through tubes 22 and 26. The final result of this is that the pulse developed by the pulse generator 22-l occurs at a time corresponding to a position nearest the beginning of a saw-tooth wave, hence advancing the timing of the direct current pulse in the output of the pulse generator 22 to its extreme position. This advanced pulse will remain in this position for as long as the ringing 20 cycle current is applied to the particular channel.

The clipper-limiter 24 eliminates variation of the amplitude of the modulated pulse (note line I0,` Fig. 3) which appear at point 68 and also discards or refuses to pass the negative unmodulated pulse of line |08, Fig. 3. This limiter prevents the passage of low amplitude pulses which would contribute to cross modulation.

The output from the clipper-limiter 24 is represented by line |09, Fig. 3, which indicates the anode current of the clipper-limiter 24 of channel #4. The dotted pulses in line |09 indicate the time variations in the pulse in this channel in response to modulation. Obviously, there will be pulses like |09 for every channel at different positions along the time base, and these pulses in the different channels do not overlap.

All output pulses from the individual channels to 8, inclusive, are applied to lead 3|, which connects with the common differentiator and limiter circuit 32. Lead 3| is connected to a differentiator coil 19 which produces small duration pulses of positive and negative polarities from the channel pulses on lead 3| as illustrated in line I0. The positive portions of these pulses appearing in line ||0 of Fig. 3 occur at the front edges of the applied pulses represented by line |09 in Fig. 3. These differentiated pulses from coil 19 (Fig. 3, line ||0) are applied to a top and bottom normally non-conducting clipperlimiter tube 80. The resulting pulses of current in the output of tube are fed into another diiferentiator coil 19', and the output from this coil 19 fed to another top and bottom normally non-conducting clipper-limiter tube 80 of circuit vCoupling tube 83 is positive .this `particular time.

..pliier.stages.-

34. Coils 19 :and 1.19" and 4tubes .80 and 80', rer spectively, lfunction in a similar manner. .The difierentiatedpulses from `coi-l 19 which are impressed on the Vgrid -of the clipper-limiter tube 88 are shown inline :III ofFig. 3. Y f

The discharge pulse which is taken toff 'the counter I4 and applied `to lead I5 .isshown .in line :I-I2 of Fig. 3. yThe positive section vof the discharge pulse initiates the discharge -of the step wave y(Fig. 3 line .I-UZ) via tube E of counter circuit. The length of -this discharge pulse lis made adjustable by means of the RC -constants of the circuit elements associated-with `.tube B in the counter circuit. This discharge pulse is applied `via lead vI5 to the-grid ofjvacuum Atube EI through diiferentiating condenser P.

The operation of the synchronizing pulse generator is :as follows: AOn the ,negative going portion of the discharge pulse (line -I I2 of Eig. 3.) tube-8l .is-cut off and'remains cut oft' A'for a period of time determined by the timegconstants of condenser P and y.resistor Pk. `(In the actual circuit tried out'in practice this time wasmade 2 .a sec.) During the time when tube 8| is cut off -a positive pulse is developed at point v82.. It

will vnow be seen that fthe occurrence time of the pulse at 82 4is a function Vof lthe length of the discharge pulse of line I-I-2, Fig. v3, and the length of the -pulse at 82 is a function of "the time constants of yP and P.v then the synchronizing pulse -and is coupled .to the output transformer T4 via-coupling tube `83. also atop and bottom limiter.

The positive going-edge of the discharge-pulse .(-line .-I I2) has no effect onytube 82 lgoing edge -P -is -charged up via. grid .of ytube 8| and hence discharged-on the negative or falling `edge of the discharge pulse.

The generated Vpulse at ypoint 8-2 is applied to the grid of coupling tube .83, which .is `normally :non-conducting due -to -the {grd leak bias `provided by resistor 85 .and condenser :84. This positive pulse `on the anode :of `tube 8I causes .tube -8-3 A'to conduct and produce a pulse of current --of -the -same shape as thevoltage pulse .applied to .tube 83. Tube A83, it should be noted., draws its current ,through .one winding of the transformer T4, Whch'is common to yboth tubes 80 and y8-3. These `common-currents in tubes 8Il 'and 8.3 give `rise to .a pulse -of -voltage in the .output-of transformer .T4 v,at point 86 `whose appearance is represented :in -li-ne II4 .of Fig. 3. Line -I I'4, Fig. 3,.-illustrates .the appearance -o f all -the channel and synchronizing pulses which occur in the output .of transformer .It will be seen that this synchronizationpulse `produced by the synchronizing pulse generator is of .longer `duration vthan the individual -channel `pulses 'and periodically occurs after every eighth channel pulse. Vacuum tubes -80 and S-ofthe -differ- --entiators and .limiters -3-2, -34 arev both normally .non-conducting. .Tube V8G' draws current each time 'a channel pulse is to be passed and `coupling tube 83 draws current only when .the synchronization Ipulse .is .to be passed. When tube Bwclraws current, tube 88 .is not conducting at .A comparison of thepulses .inlines II-I ,-and- \I.I4 .of Fig. `3 will show thatin line III there .is a blank-space for .the synchrovnization .pulse which is later lled'in in line I I4.

The output .from transformer T4 `is passed ,over .a transmissionline '8.1 to -a poweramplifer `36 compri-singthree cascaded 'vacuum .tube amy The output .ofthe poweramplier '36'.at point B8 is Yshown .inline .II-.5,..Fig. 3 .and

ysince lon the 'f12 .is applied to the @cathode of a magnetron .oscillater 4.8 -whose `anode is grounded. The `,pulses at point 88 are direct current {pulses-.of .negative polar-ity having a time duration -of .6 ,usecg for the signal pulses :and 2.2 ,L sec.. for the vsynvchronizing-pulses. These direct current Apulses are .applied to the cathode of the magnetron Aand cause the magnetron to produce radio -freyquency pulsesof a duration somewhat j-lessthan the .duration zof the pulses applied to the -magnetron. .The output .from'themagnetron -isfapplied to a suitable antenna lcircuit via a -concentric .transmissionline-89- .There is Ashown fa Lsuitable regulator circuit .for .stabilizing pulse amplitude applied -to the .magnetron `and also :a .line .coupling circuit-in the output -of the magnetron for correctly-loading the magnetron for tmaximum power output.

The radio frequency pulses from the vmagne- .trcn,=whicli :are applied -to the line -8-9 and hence to the antenna, may :have a .-freduencyof .the 4order vof 1000 'megacycles or higher, vandthe duration of these pulses are somewhat shorter than thetrig-gering-or ring pulses applied there- .to Iand-.cc eurring-at point .83. The reason that the radio :frequency pulses from the magnetron are of shorter duration :than the direct-current pulses applied thereto Tby the power amplifier .-is that the-magnetron does fnot oscillate until ,the crests of the applied pulses :are .almost reached. The radio 'frequency pulses may .have .a l.duration of about .4 .p sec. for'the signal :pulses .and :27.9 aseo. -for the synchronizing pulses.y y The'following'description relates to the .receiver and associated circuits. j

#The pulses which .the receiving '.systemgof Fig.

Il 'is designed toV receive are short .spaced pulses be in the form illustrated in curveA of ,-Fig. 5a`

which :shows eight :channel .pulses of -short .dura- .tion and one synchronizing pulse -oflonger Yduration -for Aeach frame -or cycle. The showing -of curve vA, Fig. -5a is 4for the lcondition of no modula- -tion and `is given by Way of rexampleonly. It should Iloe-noted thatthe pulses are-evenly spaced from leach other.

The .pulses of cur-ve vA, Fig. 5a, `are --collected by antenna I9 and Yimpressed-upon superheterodyne .receiver-12D, :Fig 4, Ithe output of which is inthe form of video l.pulses .which may take Athe .form -shown in curve B -of Fig. 5.a. These vdeo pulses are impressed via lead |01 upon a common unitshown as va box `in dash-lines, and also via lead --IOI, .to the different channel units of which there are eight, inthe assumption-that the system -describes an eight channel multiplex system.

The video pulses which are impressed upon the kcommon unit are .first applied to a synchronizing .pulse .separator circuit I, 2 wherein only the synchronizing pulse is effectiveto produce a pulse .which is applied to a pulse generator circuit 3, -4, 5. This synchronizing pulse separator circuit I, 2 suppresses the channel pulses because .of the fact that the channel pulsesare of shorter duration ythan the -synchronizing pulse. .I n effect, the synchronizing pulse 4separator `circuit l'comprisesa `pairof vacuum .tubes I, 2 of which vacuum tube AI serves-as a limiter.

The pulse generator 3, 4, 5 comprises three .vacuum tubes whose functions will appear in more vdetail 4later on in `connection with thefde- .scription of Fig. 4oa. rlhis pulse generator in eiect, is a discharge pulse generator which produces a discharge pulse which is sent over two paths, one of which extends to the phasing trigger circuit 6, 'I and the other of which extends via lead to a step wave generator I2.

The phasing trigger comprises two vacuum tubes E, 1 which provide a pulse of adjustable phase for driving a 9U kc. exciter vacuum tube 8. This tube has as its output, a circuit 8 which is tuned to 90 kilocycles. The output of the 90 kc. exciter, as derived from the tuned circuit therein, is in the form of a 90 kc. sine wave, which is fed to a 90 kc. limiter 9, I0. This limiter comprises a pair of vacuum tubes 9, I0 which serve to convert the applied 90 kc. sine wave to peaked positive pulses, in turn, applied via lead |05 to the step wave generator l2.

The step wave generator comprises several vacuum tubes which function to produce a step wave voltage having a plurality of steps or risers corresponding in number to the number of channels of the system. The discharge pulse applied from the discharge pulse generator 3, 4, 5 via lead |00 to the step wave generator serves to terminate the step wave voltage after a desired number of risers or steps.

The output from the step wave generator is passed'through a coupling vacuum tube I3 and then via lead I 02 to the various channel selectors 2|, 2|', 2|" of the different channel units, it being understood that there are as many channel units as there are channels in the system; in this case eight channels.

The channel selectors in the diierent channelsv are differently biased so as to become conductive on different risers or steps of the applied step wave voltage. The output of each channel selector is applied to a trigger circuit 23, 24 which is in turn associated with a gate 22, they later obtaining the signal or video pulses from lead |0|. The arrangement of the channel selector trigger circuit and gate are such as to convert the applied signal video pulses Whose times of occurrence vary in accordance with the signal modulation to variable width pulses Whose duration or Width correspond to the signal modulation.

The variable width pulses from the output of the trigger circuit 23, 24 are supplied to a low pass lter 34 which removes all high frequency components and passes the audio signal to the audio amplier tube 21.

A ringing circuit 25, 26 is also provided for calling the attention of the attendant when it is desired to apply ringing currents to aparticular channel.

From the foregoing, it will be seen that the receiving system has amplified the weak radio frequency pulses, converted them to D. C. pulses (video) and then supplied these pulses to the receiving multiplex at a suitable amplifier level. The receiving multiplex system regenerates the synchronizing period (100 microseconds) directly from the incoming signal, and with it separates the channel pulses in the same order that they were originally generated. The separated pulses are then demodulated to give eight voice frequency outputs.

A description will now be given of Figs. 6a and 6b which illustrate the circuit details of the system of the invention.

The synchronizing separator comprises a triode tube I normally biased to cut off and also tube 2 which is coupled to the output of triode I through a diierentiator circuit. Tube I acts as a limiter.

The video pulses (curve B) from the radiovreceiver are applied to the grid of triode Tube I 1s biased to be normally non-conducting and conducts only during the presen-ce of a pulse, or, putting it in other words, it conducts only during the time of the individual pulses, whether channel or synchronizing pulses. Tube acts as a limiter and its output comprises spaced pulses Whose polarity are reversed relative to the input pulses. These reversed polarity pulses in the output of tube I are differentiated and applied to the grid of tube 2. (Note curve C.) Tube 2 is valso normally non-conducting, and is so biased that only the peaked positive-going impulse produced from the trailing edge of the synchronizing pulse will cause it to conduct. This is because the peaked impulses produced from the synchronizing pulses are of larger amplitude than the peaked impulses produced from the channel pulses, due to the fact that the synchronizing pulse is of longer duration than the channel pulses.

The discharge pulse generator comprises tubes 3, 4 and 5. The output from the synchronizing separator, taken from the cathode of tube 2, is a peaked pulse in the positive direction which gradually diminishes over about 33 microseconds, and this gradual diminution of the pulse is caused by the discharge time of the capacity to ground through the high cathode resistor 40 of tube 2 and the grid of the following tube 3 in the discharge pulse generator. (Note curve D.)

Tube 3 is normally non-conducting and the application of the positive pulse thereto from the synchronizing separator causes it to conduct, as a result of which a negative pulse of sloping trailing edge is produced on its anode (curve E) which causes a negative pulse (curve F) to be supplied to the grid of tube 4. Tube 4, however, is normally conducting and the application of a negative pulse thereto causes it to cease conducting for a short period of time, thus producing a positive output pulse in its anode circuit of about 3 microseconds which looks like curve G. Tube 5 is a coupling tube and is normally non-conducting. The application of a positive pulse to its grid from the anode of tube 4 produces a discharge pulse on its cathode of positive polarity which looks like curve H.

The discharge pulse from the discharge pulse generator is supplied both to the phasing trigger and to the step wave generator. The phasing trigger is a self-restoring trigger circuit having one degree of electrical stability and comprises tubes 6 and l. In effect, this phasing trigger is nothing more or less than a delay circuit for delaying the pulses passed thereto by a predetermined amount. Normally, tube 6 is non-conductive and tube 1 conductive. When this trigger circuit is red, it remains in its active state for a time determined by the time constant of the circuit. The amount of delay is adjusted by means of a tap on the anode resistor of tube l. The positive pulse from the cathode of tube 5 of the pulse generator is applied to the grid of tube 6 cf the phasing trigger and res the trigger circuit to its active state. Output from the phasing trigger is taken from the common cathode circuit and is primarily a negative pulse whose trailing edge has an overshoot or positive peak, so to speak, which is utilized to excite the kilocycle exciter 9 (note curve J).

The 90 kilocycle exciter comprises a vacuum tube 8 having in its anode a parallel tuned circuit 8 Whose resonant frequency is 90 kilocycles. This tuned circuit is loosely coupled to the anode of the exciter tube. The exciter tube 8 hasy a selfbias which adjusts itself so that it passes current only on the overshoot or peaks or" ,the waves supplied to it by the phasing trigger. Note'curve J. This overshcot occurs once each frame; forput ting it in other words, once foreach synchronizing pulse but not necessarily at vthe synchronizing pulse time. When the exciter tube passescurrent, it supplies a driving pulse ,or kick (curve K) to the 90 kllocycle-tuned circuits. This-driving pulse or kick Ysets the tuned circuit intol oscillations at a 90 kilocycle rate. These oscillations are of sine Wave form and tend to decay slightly.` I n this particular case, the driving pulses from the 90 kilocycle exciter occur at intervals of 100 microseconds corresponding to a rate of 10,000 per second. Thus, one driving pulse is supplied to the tuned circuit for every nine cycles of sine wave oscillation generated in the tuned circuit. This is shown in curve L.' k v The portion X of the curve J is adjustable by adjustment of the resistor I8 of the vphasing trigger.

The output of the tuned circuit 8 is passed to the grid of vacuum tube 9 of the 90 kilocycle limiter. This tube passes only the positive halves of the sine wave cycles (curve L) generated by the tuned circuit, as shown in curve M, and passes these positive halves on to the cathode of vacuum tube I of the 90 kilccycle limiter. It should be noted that one pulse in each nine pulses passed by tube 9 totube vl0 is of larger amplitude than the others in each frame. This larger pulseimmedlately follows the driving pulse from the 00 kilocycle exciter; and the decaying amplitude or" the Apulse of lcurve M is causedby the inherent dampingv of the 90 kilocycle tuned circuit 8. Thus, this 90 kilocycle tuned circuit 8 is maintained in oscillation by its exciting pulse.

, Tube l0 of the 90 kilocycle limiter converts the pulses of waveform M to more peaked pulses of wave form N. The negative going pulse of curve N is caused by the reaction from the step wave generator which is coupled tothe output of -tube i0 of the 90 kilocycle limiter.

Output from the 90 kc. limiter is taken from the anode circuit of tube |0 via lead |05 and this output of curvel N is passed onto the grid of vacuum tube |2 of the step Wave generator as a series of recurring positive pulses. generator is of the form described in copending application Serial No. 629,169 led November 16, 197.45, now U. S. Patent 2,474,040, granted June 2, i949, wherein the legend positive input'pulse in the drawing thereof, corresponds to the pulses passed by the 90 kilocycle limiter 9, I0 to the step wave generator. The legend synchronizing pulses in U. S. Patent 2,474,040 corresponds to the .discharge pulse passed to the step wave generator directly from `the cathode of tube 5 of the pulse generator over lead |00. v

The coupling vacuum tube I3 corresponds to the cathode follower tube G of U. S. Patent 2,474,040. Step Wave output is taken from the cathode of this coupling tube and supplied via lead |02 to the channel selector circuits 2|, 2|', 2|'-'y etc., of the different channel units shown in Fig. 4. This step wave output supplied to the channel units is curve P. The channel units also have supplied thereto the video pulses iromthe radio receiver over lead |0|.

In thev step wave generator, the anodes o vacuum tubes |2 and |3 are connected to a common source of anode polarizing potential i-290 volts. Triodetube' |2 is biased bythe ref The step wave sistor-condenser combination 4| in its cathode A 'as va result of which an incremental chargeof voltage is built up on this condenser whenever tube l2 conducts. A step wave Voltage is built up acrosscondenser 44, and this step wave voltage has a. plurality of steps of increasing amplitude corresponding in number to a desired number of input pulses (curve P).

Tube isa normallyinon-conductive vacuum tube which, 'whenV it conducts,4 serves torestore coupling condenser'43 to its original condition afterr each input pulse. The cathodeof tube` is connected to the grid of tube |2, While-the grid of tube is connected via lead 46 to the circuit combination 4|. ,l

Tubes a., b and c are normally non-conductive and becomev conductive simultaneously. The grids of these tubes are connected together and their cathodes arealso connected together-and to ground. `The space path of tube c is effectively across step condenser 44, and when this tube conducts, it forms a low impedance discharge path across condenser 44. Tube b, when it conducts, insures the quick discharge of the load capacity connected Ato thev cathodel of tube |3. Tubea serves to prevent tube l2 from conducting during vthe discharge of step condenser 44.

In Fig'. 4, the channel selectors 2|, 2|', 2|" are differently self-biased, and each channel selector is normally biased well beyond the current cut-01T condition.

The channel selector is substantially like that shown in the transmitter, Fig. 2 except for the fact thatin the-anode circuit, a coil 30 is used instead ofA a resistor.

a much shorter. pulse than that provided by the outputof the channel selector' described in the Houghton copending application. The channel selectors are differently biased to operate on successive risers of the step wave output- (curve P) from the step wave generator 12. Each step or rise in thestep wave voltage appearing in lead' |02 isfgreat enough to insure that duringits occurrence the current of the correspondingbiased channel: selector shall lbe driven rapidly` from beyond the current cut-off condition to azero bias Value'. VOnce a channel selector, which is normallycut-off, passes current, (it responds to the increase vin voltage on the particular step rise) it will continue to pass current for the completion'of the step wave voltage. The differentiated short pulse outputfrom the channel selector 2| is in the negative direction, as shownin curve Q, for example, (corresponding to channel 4) and is passed on to a'self-restoring triggercircuit comprising vacuum tubes 23 and24.

Tube 23 of the trigger circuit isv normally conducting While tube 24 is non-conducting. The trigger circuit has one degree of electrical stability and has a stable state and an active state. Its time constant isr such that it would normally restore to normal (the stable state) if left alone after it has been fired to the active state, restor- This coil 30, Fig. 6b is a f differentiating coil and its purpose is to provide ing itself after a time interval considerably longer than the time allotted for one channel.

However, the trigger circuit is made to restore itself to normal by the video signal pulse supplied to a gate 22 at a time interval after being red which is. considerably shorter than the time allotted for one channel. Putting it in other words, the channel selector 2| res the trigger circuit to its active state, while the gate 22 turns oif vor restores the trigger circuit to its stable state. The output from the trigger circuit is thus a pulse of variable width, depending upon the time interval between the channel selector output pulse and the incoming video signal pulse for that channel. Thus, it will be seen that the channel selector 2 l the gate 22 and the trigger circuit 23, 24 are in eiect a system for transforming the time-displaced video signal pulses to variable width pulses, the variation in width corresponding to the signal modulation at the remote transmitter which varies the timing of the transmitted pulses. It should be understood, of course, that I am now considering the pulses for a single channel, inasmuch as the different channel units have similar equipment for effecting the same results for their own respective channel pulses.

The video pulses from the output of the radio receiver are applied over lead IBI to the grid of the gate 22. Gate vacuum tube 22 is normally non-conductive. These video pulses are of positive polarity and are not of themselves of sufficient amplitude to make the gate 22 conduct. However, when the trigger circuit 23, 24 is red and is in its active state, there is supplied to the grid of the gate 22 over lead 3l a positive voltage pulse from the common cathode circuit of the trigger circuit. This positive voltage pulse from the trigger circuit applied to the grid is of suicient magnitude to reduce the cut-oit bias on the tube 22 to an extent such that if at this time a video signal pulse of positive polarity is also applied to the grid via lead IGI, the resulting voltage from both the video signal pulse and the trigger pulse are sufficient to overcome the bias on the gate 22 and cause it to conduct. It will thus be seen that only in the simultaneous presence of both the video signal pulses and a positive pulse from the trigger circuit will the gate 22 of any one channel pass current. When the gate 22 passes current, the voltage on its anode will drop and cause the effective application via lead 32 of a negative pulse to the anode of tube 23 of the trigger circuitoi a magnitude suiiicient to restore the trigger circuit to normal. Output from the trigger circuit in the form of a variable width pulse is taken from the anode ofk tube 24 via lead 33 and passed onto a low pass lter 34. This low pass lter removes all high frequency components and passes the audio signal to the audio amplier tube 21.

Curve R illustrates the voltage wave form appearing on the grid of gate 22. The short sharp pulses correspond to the signal pulses and these short sharp pulses are not sufficient of themselves to overcome the cut-off bias of the gate. The large, longer duration pulse corresponds to the voltage occurring on the grid of 22 due to the simultaneous application of both a video signal pulse and a positive pulse from the trigger circuit, and the peak of this larger amplitude longer duration pulse has a magnitude sufficient to overcome the cut-ofi bias on the gate 22.

Curve S shows the voltage variation at the common cathode of the trigger circuit 23, 22. The fixed edge is determined by the iiring pulse 18 for the trigger circuit 23,24 supplied fromthe channel selector, while the trailing or modulated edge is determined bythe restoring pulse supplied by the gate 22.

Pulse rectifier tube 25 and rectifier tube 26 comprise the ringing system. The rectifier 25 has an input pulse supplied to its grid from the anode of tu-be 23 of the trigger circuit through a low pass filter comprising a high resistance 35 and a capacity formed by the grid-to-ground capacity of tube 25. The pulses from the trigger circuit, after passing through the low pass filter, appear on the grid of tube 25y in the form shown in curve T.

In the absence of signal modulation for this particular channel, the wave form at the gridy of tube 25 has the form of the solid line of curve T. Pulse rectiiier tube 25 is supplied with a floating xed bias from the rectifier 26. This bias is adjusted to a Value such that the pulse of the solid line of curve'T causes conduction near the peak of this pulse. This conduction current ows through resistance 36 in the cathode of tube 25`to a source of negative bias -10 Volts; The conduction current through resistor 35 is suflicient to maintain the cathode of tube 25 at a potential in the neighborhood of +5 volts with respect to ground.

This bias is supplied to the grid of audio amplifier tube 21 through a low pass filter comprising resistances 31, 38 and capacitor 39. This condition causes audio amplifier tube 21 to conduct in normal fashion as an audio amplifier. Y

In the presence of modulation, the pulses applied to the grid of tube 25 are shown in their extreme modulated positions by the dotted lines of curves T. It should be kept in mind that the positive going pulse appearing at the anode of trigger tube 23 has a variable width corresponding with the channel modulation. This variable width pulse undergoes a wave form distortion upon passing the low pass filter between the anode of tube 23 and the grid of tube 25.v This distortion is such that a wide pulse is passed with a small amplitude, as shown in curve T. It will be observed that tube 25 conducts only on pulses greater than a certain amplitude near the peak value of the pulses shown in curve T and hence conducts during these pulses with or without the presence of modulation; thus, audioamplier tube 21 remains in its conducting condition.

As described hereinabove in connection with the transmitter, the ringing is accomplished by advancing a channel pulse to its extreme position, where it remains for the duration of a ring. This ringing condition produces -a narrow pulse at the anode of trigger tube 23 and this pulse', after passing the low pass filter to the'gr'idof tube 25, is of such low amplitude thatit cannot overcome the nxed bias on tube 25; hence tube 25 remains completely non-conducting. Under this condition, the Voltage at the cathode to tube 25 is maintained at a value close to minus 10 volts with respect to ground. This condition provides this same negative bias at the grid of audio amplifier tube 21 causing it to be essentially at cut-oil. In the plate circuit of tube 21 is connected, along with the audio output transformer, a ringing relay which is kept closed by the normal plate current of this tube. When this tube is cut off, the ringing relay opens and causes a twenty cycle ringing channel generator to be started and switched on to the outgoing line.

What is claimed is:

1. A multiplex communication system of the l9` typev wherein a plurality of'cha'nnels are coupled `toa common Vtransmission mediumduring different sequentially-assigned time intervals,A including a channel selector for each'of vsaid channe1s,'a generator to produce afrecurring step- Wave voltage havingv a plurality vof steps of different voltage values; all of said channel selectors being connected in parallel to said generator to receive said recurring step-'Wave voltage, the bias on said channel selectors being different to render said selectors efective'at different steps of the received step-wave voltage,y each channel selector having a pulse generator coupled thereto for control thereby, an output circuit common to said channels,l differentiating land clipping circuitry interposed in said 'common output circuit to shorten the effective lengths of the pulses therein, a synchronizing pulse'. generator coupled to said recurring step-wave voltage generator and repensive to the recurrence of the 'step-Wave to produce a synchronizing pulse, and a circuit common to said pulse 'generator and said channels to combine the synchronizing pulse with theroutput from said channels.

y2'. In' a multiplex communication system wherein different channels are coupled to -a common transmission medium during different sequentially-assigned time intervals, each of said channels having a channel selector, a recurring step-wave voltage 'generator coupled in parallel to the input circuits of the different channel selectors to apply a recurring step-wave voltage having a plurality of steps of different voltage values thereto, said channel selectorsbeing Ydifferently' biased to lbecome effective at different stepsV of the applied'step-Wave voltage, a pulse generator coupled tothe channel selector of each channel for' control by 'the channel selector thereof, an outputl circuit common to all of said channels,'diierentiating andl clipping circuitry interposed in said common output circuit to shorten the effective lengths of the pulsesV produced therein, a synchronizing pulse generator coupled to said'recu'rring step-Wave voltage generatorand responsive to the recurrence of the step-Waver to produce a synchronizing pulse, and a circuit common to said synchronizingp'ulse generator and allot said channels to combine the 1synchronizing pulse with the pulses from said channels.

" 3; Av multiplei communication system ot the type wherein a plurality of different channels are sequentially connected to a'common transmission mediu i, 'said channels each having signal input andsignal output circuits, including a channel selector in each channel normally rendering said channel inoperative, eachl of said channel selectors being biased to render the associated'channel operative ata 'predetermined biasl different 20 from that for all other of said channelathe 'output circuits'of said channels being connected in common, a local wave generator arranged to Vproduce `a recurring step-Wave having a plurality of steps of diierent voltage values, said local wave generator being connected to all .of said channel selectors to apply saidy recurring step-wave voltages thereto to render said channels operative at diierent steps of said Wave, 'a synchronizing pulse generator coupled to said local Waveigenerator to derive a synchronizing pulse in response to the recurrence of said voltage step-wave, and circuit connections between said pulse generator and said common output circuit to combine the synchronizing pulse with the signalv output of said channels.

4. In a multiplex communication system wherein a common transmission medium is sequentially assigned to differentV channels whereby the different channels are coupled to said 'transf mission medium during different time intervals, said channels each having -a channel selector; a step-Wave voltage generator supplying the inputs of the dierent channel selectors in parallel with a recurring step-Wave voltage having a plurality of steps of different voltage values, said'step wave generator having means -for dischargingthe step voltage Wave when the amplitude of said'wave reaches a predetermined value, said-channel selectors being differently biased to become effective at diierent stepsor-risers of the applied stepwave voltage, a pulse generator in each channel under control of the channel selector therein, a common output circuit for all of said channels, a diiferentiator and clippercircuit in said common output circuit for shortening the eective lengths of the pulses therein, a synchronizing pulse generator coupledl to said step-Wave voltage generator andv responsive to the discharge-of the step Wave vfor producing a synchronizing'pulse, anda common amplifier for combining the synchronizing pulse with the pulses from s aid 'channe s.

REFERENCES CITED The following references are" of'recordi'n the le of this patent:

UNITED` STATES Pri-'rinitisl 

